In general, a phosphor dot layer of three colors, that is, blue, green, and red, is regularly formed on the inner surface of the front panel of a color cathode ray tube by a photographic method. A light absorption layer, which is referred to as a black matrix, is formed around the phosphor dots of three colors to improve the contrast. When forming the phosphor layer and the black matrix on the inner surface of the front panel, an exposure system for manufacturing a color cathode ray tube is used.
FIG. 8 shows a schematic diagram of an exposure system for manufacturing a color cathode ray tube. As shown in FIG. 8, the exposure system comprises a light source portion 9, a correction lens 10, and a light quantity correction filter 11. Light irradiated from the light source portion 9 passes through the correction lens 10 and the light quantity correction filter 11 and exposes a phosphor screen on the inner surface of a front panel 12. The light quantity correction filter 11 corrects the light quantity distribution of the light that passed through the correction lens 10 from the light source portion 9 to make it uniform on the inner surface of the front panel 12.
Methods for correcting the light quantity distribution of the light irradiated from the light source portion by using a light quantity correction filter having the above function include providing a predetermined light transmittance distribution by using a light quantity correction filter in which a plurality of stripe-shaped shielding portions are provided and in which the area ratio of the shielding portions to light transmission portions between the shielding portions is adjusted by the width of the shielding portions, as disclosed in Japanese Patent Application (Tokko Sho) No. 58-43853. Another method is, for example, to control the light transmittance by using a light quantity correction filter comprising an evaporated film, as disclosed in Japanese Patent Application (Tokkai Hei) No. 6-103895.
The light quantity correction filter comprising an evaporated film is manufactured, for example, by using an apparatus as shown in FIG. 9. FIG. 9 shows a schematic diagram of an apparatus for manufacturing a light quantity correction filter according to the prior art. A shielding sheet (an evaporation pattern) 13 having an opening 14 is provided between a transparent substrate 3 and an evaporation source 2. In this apparatus, the desired transmittance distribution for correction is obtained on the transparent substrate 3 by rotating the shielding sheet 13, changing the rotational speed according to the angle, and controlling the evaporation amount by the time integration ratio of the opening to the shielding portion.
Recently, in high-definition color monitor display tubes and color cathode ray tubes for high-definition televisions, reproducibility of colors and a white uniformity have been precisely required along with a large size, a wide angle deflection, and the flatness of the screen surface. Therefore, a complicated highly precise exposure correction is required during the process of forming a phosphor screen, especially the process of forming a black matrix. Accordingly, the effect of the collection and divergence of the lens becomes complicated, and a transmittance distribution for performing the corresponding complicated highly precise correction is required for a light quantity correction filter that controls the size of a back matrix.
With the correction method using the light quantity correction filter comprising a plurality of stripe-shaped shielding portions according to the prior art, however, a mechanism for oscillating the light quantity correction filter should be provided in the exposure system. Therefore, the structure of the exposure systems, a large number of which are used in the process of manufacturing color cathode ray tubes, becomes complicated. With the method of making the light quantity correction filter comprising an evaporated film according to the prior art, the evaporation amount is indirectly controlled by the time integration ratio of the opening to the shielding portion that depends on the shape of the opening, the rotational speed, and the like. Therefore, the attainable transmittance distribution is limited, and the control of the transmittance distribution is very complicated. Consequently, it is very difficult to provide the desired black matrix size distribution with a tolerance of 5% or less and obtain a sufficient white quality.